spin temperature

A way of measuring the temperature of neutral hydrogen
(HI) in space in terms of the relative populations of its two possible spin
states – parallel (electron and proton spins aligned) and antiparallel
(spins opposed). The spin temperature of an HI region
is the temperature at which its observed ratio of parallel to antiparallel
spins would occur if the gas were in thermal equilibrium. It differs from
the kinetic temperature at low
densities, where the spin temperature is raised by the tendency of resonant
scattering of Lyman-alpha photons to equalize the occupation probabilities
in the two spin states.